Nitrogen Dioxide Absorption in Evaporating and Condensing Water Droplets

摘要

A NO sub 2 absorption enhancement under conditions of water condensation may be that there is a net movement of water vapor toward the water surface, tending to drag extra NO sub 2 molecules along, whereas in the steady-state condition there exists an equimolar exchange of water molecules both to and from the surface. The results show that, as H sub 2 O condensation ceases, the NO sub 2 concentration level rapidly returns to saturation. Mechanisms which may promote this phenomenon are: (a) as evaporation takes over the outer layers of solution are stripped away, releasing NO sub 2 back into the atmosphere; (b) dissolved NO sub 2 , present either as NO sub 2- or NO sub 3- may become too concentrated in the outer layers and revert to NO sub 2 and then escape as a gas; (c) evaporating water molecules in the vicinity of the interface may reduce the gas phase NO sub 2 concentration to a level below that in the bulk gas, providing an extra driving force. The fact that the desorption is so rapid indicates that any one or a combination of the above mechanisms are acting more rapidly than the combined liquid phase diffusion-chemical reaction mechanism tending to absorb the NO sub 2 into the droplet interior. (ERA citation 04:024489)